Chemically strengthened glass

ABSTRACT

A chemically strengthened glass having a compressive stress layer formed in a surface layer thereof according to an ion exchange method, in which the glass has a surface roughness (Ra) of 0.20 nm or higher, a hydrogen concentration Y in a region to a depth X from an outermost surface of the glass satisfies the following relational equation (I) at X=from 0.1 to 0.4 (μm), a surface strength F (N) measured by a ball-on-ring test under the following conditions is (F≧1500×t 2 ) relative to a sheet thickness t (mm) of the glass, and a surface of the glass has no polishing flaw:
 
 Y=aX+b   (I)
 
in which meanings of respective symbols in the equation (I) are as follows: Y: hydrogen concentration (as H 2 O, mol/L); X: depth from the outermost surface of the glass (μm); a: −0.270 to −0.005; and b: 0.020 to 0.220.

TECHNICAL FIELD

The present invention relates to a chemically strengthened glass.

BACKGROUND ART

In flat panel display devices such as digital cameras, mobile phones,personal digital assistants (PDAs), etc, in order to protect displaysand enhance the appearance thereof, a thin plate-like cover glass isdisposed on the front surface of the display so as to provide a broaderregion than an image display portion. Although the glass has a hightheoretical strength, when scratched, its strength is largely lowered,and therefore, for the cover glass that is required to satisfy strength,a chemically strengthened glass having a compressive stress layer formedon the surface thereof through ion exchange or the like is used.

With the growing demand for weight reduction and thickness reduction inflat panel display devices, it is desired to thin cover glass itself.Accordingly, for satisfying the purpose, both the surfaces and the edgesurfaces of cover glass are desired to have further strength.

For increasing the strength of the chemically strengthened glass,heretofore, a surface etching treatment to be conducted after chemicalstrengthening treatment is known (Patent Document 1).

Here, regarding the strength of glass, it is known that the strength ofglass is lowered by the existence of hydrogen (water) in glass(Non-Patent Documents 1 and 2).

BACKGROUND ART DOCUMENT Patent Document

-   Patent Document 1: JP-T-2013-516387

Non-Patent Document

-   Non-Patent Document 1: S. ITO et. al., “Crack Blunting of    High-Silica Glass”, Journal of the American Ceramic Society, Vol.    65, No. 8, (1982), 368-371-   Non-Patent Document 2: Won-Taek Han et. al., “Effect of residual    water in silica glass on static fatigue”, Journal of Non-Crystalline    Solids, 127, (1991) 97-104

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

The present inventors have found that there is a concern that thestrength of glass is lowered after the chemical strengthening, and themajor cause thereof is that moisture in the atmosphere penetrates intothe glass surface layer to form chemical defects. Further, the presentinventors have found that this phenomenon occurs not only throughchemical strengthening but also through a heating step in glassproduction process.

As a technique for removing moisture from a glass surface layer, it maybe considered to chip off the moisture-containing layer according to atechnique of polishing the glass surface after chemical strengthening oraccording to a technique of subjecting the glass surface after chemicalstrengthening to an etching treatment by immersing in hydrofluoric acidor the like. However, there is a concern that the surface of glass isscratched by polishing so that the strength thereof rather lowers. Inaddition, in a case where the glass surface has latent flaws, there is aconcern that the etching treatment using hydrofluoric acid or the likegrows the latent flaws to cause appearance failure owing to pits.Further, hydrofluoric acid requires careful handling in view of safety.

An object of the present invention is to provide a chemicallystrengthened glass capable of effectively preventing the strength ofglass from lowering even after performing chemical strengthening.

Means for Solving the Problems

The present inventors have found that by not only allowing a hydrogenconcentration profile on a surface layer of a chemically strengthenedglass to fall within a specific range but also controlling a surfaceroughness (Ra) to a specific value or higher, the surface strength ofglass can be dramatically improved and a reliability of the surfacestrength can be improved, without polishing the glass surface afterchemical strengthening or subjecting the glass surface after chemicalstrengthening to the etching treatment using hydrofluoric acid or thelike, thereby accomplishing the present invention.

Namely, the present invention is as shown below.

<1> A chemically strengthened glass having a compressive stress layerformed in a surface layer thereof according to an ion exchange method,

in which the glass has a surface roughness (Ra) of 0.20 nm or higher,

a hydrogen concentration Y in a region to a depth X from an outermostsurface of the glass satisfies the following relational equation (I) atX=from 0.1 to 0.4 (μm),

a surface strength F (N) measured by a ball-on-ring test under thefollowing conditions is (F≧1500×t²) relative to a sheet thickness t (mm)of the glass, and

a surface of the glass has no polishing flaw:Y=aX+b  (I)in which meanings of respective symbols in the equation (I) are asfollows:

Y: hydrogen concentration (as H₂O, mol/L);

X: depth from the outermost surface of the glass (μm);

a: −0.270 to −0.005; and

b: 0.020 to 0.220,

the conditions of the ball-on-ring test:

a sheet of the glass having the sheet thickness t (mm) is disposed on astainless ring whose diameter is 30 mm and whose contact part has aroundness with a curvature radius of 2.5 mm; while a steel ball having adiameter of 10 mm is kept in contact with the sheet of the glass, acenter of the ring is subjected to a load by the ball under a staticloading condition; and a fracture load (unit: N) at which the glass isfractured is taken as a BOR surface strength and an average value oftwenty measured values thereof is taken as the surface strength F,provided that in a case where a fracture origin of the glass isseparated from a loading point of the ball by 2 mm or more, the obtainedvalue is excluded from data for calculating the average value.

<2> The chemically strengthened glass according to <1>, in which theglass is an aluminosilicate glass, an aluminoborosilicate glass or asoda-lime glass.

Advantage of the Invention

According to the chemically strengthened glass of the invention, by notonly allowing a hydrogen concentration profile on a surface layer of achemically strengthened glass to fall within a specific range but alsocontrolling a surface roughness (Ra) to a specific value or higher, thesurface strength of glass can be dramatically improved and a reliabilityof the surface strength can be improved, without polishing the glasssurface after chemical strengthening.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view for explaining a method of a ball-on-ringtest.

FIG. 2 is a schematic view showing a production process of a chemicallystrengthened glass according to the present invention.

FIG. 3 is a graph of plotting the hydrogen concentration profile in thesurface layer of each chemically strengthened glass obtained in Examples1 and 2.

FIG. 4 is a graph of plotting the hydrogen concentration profile in thesurface layer of each chemically strengthened glass obtained in Examples3 and 4.

FIG. 5 is a graph of plotting the hydrogen concentration profile in thesurface layer of each chemically strengthened glass obtained inComparative Examples 1, 2 and 3.

FIG. 6 is an explanatory view for deriving the relational equation (I)from the graph of plotting the hydrogen concentration profile in thesurface layer of the chemically strengthened glass obtained in Example1.

FIG. 7 is an explanatory view for deriving the relational equation (I)from the graph of plotting the hydrogen concentration profile in thesurface layer of the chemically strengthened glass obtained inComparative Example 1.

FIG. 8 is a Weibull plot of BOR surface strength evaluation of each ofchemically strengthened glasses obtained in Example 1 and ComparativeExample 1.

FIG. 9 is an AFM image of the chemically strengthened glass surface ofReference Example 1. The scanning area was 5×5 μm².

FIG. 10 is an AFM image of the chemically strengthened glass surface ofExample 1. The scanning area was 5×5 μm².

FIG. 11 is a Weibull plot of BOR surface strength evaluation of each ofchemically strengthened glasses obtained in Example 3 and ReferenceExample 2.

FIG. 12 is a graph of plotting the hydrogen concentration profile in thesurface layer of each chemically strengthened glass obtained in Example3 and Reference Example 2.

FIG. 13 is an AFM image of a glass surface having surface polishingflaws.

FIG. 14 is an AFM image of a glass surface having no surface polishingflaw.

MODE FOR CARRYING OUT THE INVENTION

The present invention is hereunder described in detail, but it shouldnot be construed that the present invention is limited to the followingembodiments, and the present invention may be arbitrarily modified andcarried out within the range where the gist of the present invention isnot deviated.

Here, in the present specification, “% by mass” has the same meaning asthat of “% by weight”, and “ppm by mass” has the same meaning as that of“ppm by weight”. Simple expression of “ppm” indicates “ppm by weight”.

<Chemically Strengthened Glass>

The chemically strengthened glass according to the present invention isa chemically strengthened glass having a compressive stress layer formedin the surface layer thereof according to an ion exchange method, inwhich the hydrogen concentration in the region to a certain depth fromthe outermost surface of the glass satisfies the following relationalequation (I), and the glass surface has no polishing flaw.

The compressive stress layer is a high-density layer formed through ionexchange of the Na ion in a glass surface with the K ion in a moltensalt by bringing a starting material glass into contact with aninorganic molten salt such as potassium nitrate.

In the chemically strengthened glass of the present invention, thehydrogen concentration profile in the glass surface layer falls within aspecific range. Specifically, the hydrogen concentration Y in a regionto a depth X from the outermost surface of the glass satisfies thefollowing relational equation (I) at X=from 0.1 to 0.4 (μm).Y=aX+b  (I)[In the equation (I), the meanings of the respective symbols are asfollows:

Y: hydrogen concentration (as H₂O, mol/L);

X: depth from the outermost surface of glass (μm);

a: −0.270 to −0.005; and

b: 0.020 to 0.220]

Regarding the surface strength of a glass, it is known that the surfacestrength of a glass lowers owing to the presence of hydrogen (moisture)in the glass, but the present inventors have found that there is aconcern that the surface strength of glass is lowered after the chemicalstrengthening treatment, and the major cause thereof is that moisture inthe atmosphere penetrates into the glass to form chemical defects.Further, the present inventors have found that this phenomenon occursnot only through the chemical strengthening but also through a heatingstep in glass production process.

When the hydrogen concentration in a glass is high, hydrogen penetratesinto the Si—O—Si bond network in the glass in the form of Si—OH wherebythe bond of Si—O—Si is cut. When the hydrogen concentration in the glassis high, it is considered that the part where the Si—O—Si bond is cutincreases so that chemical defects may be easily formed, whereby thesurface strength is lowered.

The above-mentioned relational equation (I) holds in a region of fromthe outermost surface to a depth X=from 0.1 to 0.4 μm. The thickness ofthe compressive stress layer to be formed through ion exchange fallswithin a range of from 5 to 50 μm, though it depends on the degree ofchemical strengthening. The hydrogen penetration depth into glassfollows the diffusion coefficient, temperature and time, and thehydrogen penetration amount is influenced by the moisture amount in theatmosphere in addition to these. The hydrogen concentration afterchemical strengthening is the highest in the outermost layer andgradually reduces toward the deep part (bulk) where the compressivestress layer is not formed. The above-mentioned relational equation (I)defines the profile of the reduction, and in the outermost surface (X=0μm), there is a possibility that the moisture concentration may varyowing to time-dependent degradation, and therefore the equation isdefined to hold in a region of the near surface (X=from 0.1 to 0.4 μm)not influenced by the variation.

In the equation (I), a indicates an inclination to define the profile ofreduction in the hydrogen concentration. The range of a is from −0.270to −0.005, preferably from −0.240 to −0.030, more preferably from −0.210to −0.050.

In the equation (I), b corresponds to the hydrogen concentration in theoutermost surface (X=0 μm). The range of b is from 0.020 to 0.220,preferably from 0.020 to 0.215, more preferably from 0.030 to 0.210,further preferably from 0.040 to 0.200.

In general, the surface strength reduction of a glass is considered tobe caused by growth of microcracks existing in the glass surface owingto the mechanical stress given thereto from the outside. According toNon-Patent Document 2, when the glass structure at the tip of a crack isin a Si—OH-richer state, it is considered that the cracks easily grow.Assuming that the tips of cracks are exposed out in the atmosphere, theSi—OH amount in the tip of the crack is presumed to have a positiverelationship to the hydrogen concentration in the glass outermostsurface. Accordingly, b corresponding to the hydrogen concentration inthe outermost surface preferably falls within a low range to the degreeas shown above.

As shown in FIGS. 3 to 5, the glass processed through a chemicalstrengthening step did not show any remarkable difference in thehydrogen penetration depth. There is a possibility that the hydrogenpenetration depth may vary depending on the condition of the chemicalstrengthening step, but if the depth does not change at all, thereappears a negative correlation between b that corresponds to thehydrogen concentration in the outermost surface and a that correspondsto the inclination to define the profile of reduction in the hydrogenconcentration. Accordingly, a preferably falls within a high range to adegree as shown above.

As described above, in the present invention, it has been found that,not only by defining the hydrogen concentration itself alone in thesurface layer but also by defining the hydrogen concentration in thesurface layer and the reduction profile thereof each to fall within aspecific range, with taking particular note of the hydrogenconcentration profile, the surface strength of chemically strengthenedglass can be greatly improved.

[Method for Measuring Hydrogen Concentration Profile]

Here, the hydrogen concentration profile (H₂O concentration, mol/L) of aglass is a profile measured under the following analysis condition.

For measurement of the hydrogen concentration profile of a glasssubstrate, a method of secondary ion mass spectrometry (SIMS) isemployed. In a case where a quantitative hydrogen concentration profileis obtained through SIMS, a standard sample whose hydrogen concentrationis known is necessary. A method for preparing the standard sample and amethod for determination of the hydrogen concentration thereof aredescribed below.

1) A part of the glass substrate to be analyzed is cut out.

2) A region of 50 μm or more from the surface of the thus-cut glasssubstrate is removed by polishing or chemical etching. The removaltreatment is carried out on both surfaces. Namely, the thickness to beremoved on both surfaces is 100 μm or more. The glass substrate that hasbeen subjected to the removal treatment is used as a standard sample.3) The standard sample is analyzed through infrared spectroscopy (IR),and the absorbance height A₃₅₅₀ at the peak top near 3,550 cm⁻¹ in theIR spectrum and the absorbance height A₄₀₀₀ (base line) at 4,000 cm⁻¹are determined.4) The thickness d (cm) of the standard sample is measured using athickness measuring device such as a micrometer.5) With reference to the reference A, the hydrogen concentration (asH₂O, mol/L) in the standard sample is determined using the formula II,in which the infrared practical absorbance index of H₂O in glassε_(pract) (L/(mol·cm)) is 75.Hydrogen concentration in standard sample=(A ₃₅₅₀ −A ₄₀₀₀)/(ε_(pract)·d)   Formula II

-   Reference A): S. Ilievski et al., Glastech. Ber. Glass Sci.    Technol., 73 (2000) 39.

The glass substrate to be analyzed and the standard sample whosehydrogen concentration is known, as prepared according to theabove-mentioned method, are simultaneously fed into a SIMS device, andanalyzed sequentially to obtain the depth direction profiles of theintensities of ¹H⁻ and ³⁰Si⁻. Subsequently, the ¹H⁻ profile is dividedby the ³⁰Si⁻ profile to obtain a depth direction profile of ¹H⁻/³⁰Si⁻intensity ratio. From the depth direction profile of the ¹H⁻/³⁰Si⁻intensity ratio of the standard sample, an average ¹H⁻/³⁰Si⁻ intensityratio in a region of a depth of from 1 μm to 2 μm is calculated, and acalibration curve of this value and the hydrogen concentration is drawnto pass through the origin (calibration curve with one-level standardsample). Using the calibration curve, the ¹H⁻/³⁰Si⁻ intensity ratio onthe vertical axis of the profile of the glass substrate to be analyzedis converted into the hydrogen concentration. Accordingly, the hydrogenconcentration profile of the glass substrate to be analyzed is obtained.The measurement conditions in SIMS and IR are as follows.

[SIMS Measurement Condition]

Device: ADEPT 1010 manufactured by ULVAC-PHI, Inc.,

Primary ion species: Cs⁺

Primary ion accelerating voltage: 5 kV

Primary ion current value: 500 nA

Primary ion incident angle: 60° relative to the normal line of thesample plane

Primary ion luster size: 300×300 μm²

Secondary ion polarity: minus

Secondary ion detection region: 60×60 μm² (4% of luster size of primaryion)

ESA Input Lens: 0

Use of neutralization gun: yes

Method of converting the horizontal axis from sputtering time to depth:The depth of the analysis crater is measured with a stylus surfaceprofile analyzer (Dektak 150, manufactured by Veeco Inc.), and theprimary ion sputtering rate is determined. Using the sputtering rate,the horizontal axis is converted from the sputtering time to the depth.Field Axis Potential in ¹H⁻ detection: The optimum value may change inevery device. The operator should carefully define the value so that thebackground is fully cut off.[IR Measurement Condition]Device: Nic-plan/Nicolet 6700, manufactured by Thermo Fisher ScientificCo., Ltd.Resolution: 4 cm⁻¹Number of scans: 16Detector: TGS detector

For deriving the relational equation (I) from the hydrogen concentrationprofile (H₂O concentration, mol/L) of the glass determined under theabove-mentioned analysis condition, the following procedure is employed.As shown in FIG. 6 and FIG. 7, linear approximation is applied to thehydrogen concentration profile in a region of a depth of from 0.1 to 0.4μm. The equation of the resultant approximation straight line isreferred to as the relational equation (I).

As a means of controlling a and b, for example, the fusing agentconcentration, sodium concentration, temperature and time in thechemical strengthening step may be changed.

It is preferable that the chemically strengthened glass of the presentinvention has an average hydrogen concentration c of from 0.070 to 0.150mol/L in a near-surface region (depth X from the outermost surface is0.1 to 0.4 μm). It is thought that in a case where the average hydrogenconcentration is within that range, not only the chemically strengthenedglass has a high surface strength but also the reliability of thesurface strength is improved. Incidentally, the average hydrogenconcentration c can be obtained from the hydrogen concentration profiledescribed above.

(Surface Polishing Flaws)

The chemically strengthened glass of the present invention has nopolishing flaw on the surface thereof. Here, polishing in the presentinvention means that the surface of a glass is polished with abrasivesfor smoothing. The presence or absence of polishing flaws may bediscerned through surface observation with AFM (Atomic ForceMicroscope). A case where two or more scratches each having a length of5 μm or more and a width of 0.1 μm or more are not present in a regionof 10 μm×5 μm can be said to be in a state that the surface has nopolishing flaw. FIG. 13 shows a state having surface polishing flaws,and FIG. 14 shows a state having no surface polishing flaw.

(Glass Surface Strength)

The surface strength of the chemically strengthened glass of the presentinvention can be evaluated according to a ball-on-ring test.

(Ball-on-Ring Test)

The chemically strengthened glass of the present invention is evaluatedin terms of the BOR surface strength F (N) measured by a ball-on-ring(BOR) test, in which a glass sheet is disposed on a stainless ring whosediameter is 30 mm and whose contact part has a roundness with acurvature radius of 2.5 mm, and while a steel ball having a diameter of10 mm is kept in contact with the glass sheet, the center of the ring issubjected to a load by the ball under a static loading condition.

The chemically strengthened glass of the present invention satisfiesF≧1,500×t², preferably F≧1,800×t². [In the formulae, F means the BORsurface strength (N) measured by the ball-on-ring test, and t means thethickness (mm) of the glass sheet.] When the BOR surface strength F (N)falls within the range, the glass exhibits an excellent surface strengtheven when formed into a thin sheet.

FIG. 1 shows a schematic view for explaining the ball-on-ring test usedin the present invention. In the ball-on-ring (BOR) test, a glass sheet1 is, while kept set horizontally, pressurized by a pressurizing jig 2made of SUS304 (hardened steel, diameter: 10 mm, mirror-finished) tomeasure the surface strength of the glass sheet 1.

In FIG. 1, the glass sheet 1 to be a sample is horizontally set on areceiving jig 3 made of SUS304 (diameter: 30 mm, radius of curvature ofthe contact part R: 2.5 mm, the contact part is hardened steel,mirror-finished). Above the glass sheet 1, a pressurizing jig 2 forpressurizing the glass sheet 1 is arranged.

In this embodiment, the center region of the glass sheet 1 obtained inExamples and Comparative Examples is pressurized from above. The testcondition is as mentioned below.

Descending Rate of Pressurizing Jig 2: 1.0 (Mm/Min)

In this test, the fracture load (unit: N) at which the glass isfractured is taken as a BOR surface strength. The average value oftwenty measured values thereof is taken as a surface strength F.However, in a case where the fracture origin of the glass sheet isseparated from a loading point of the ball by 2 mm or more, the obtainedvalue is excluded from the data for calculating the average value.

The chemically strengthened glass of the present invention not only hasa high surface strength as described above but also has high reliabilityof the surface strength. As shown the Weibull plots of the BOR surfacestrength evaluation of chemically strengthened glasses shown in Exampleswhich will be described later, it can be seen that the chemicallystrengthened glass of the present invention has little unevenness insurface strength. Although the reason therefor is unclear, it ispresumed that the reduced unevenness in surface strength is due to thehydrogen (moisture) concentration which is slightly high in the vicinityof the glass surface layer.

(Surface Roughness)

The chemically strengthened glass of the present invention further has asurface roughness (Ra) of 0.20 nm or higher. Since the surface roughnessthereof is not less than that value, a chemically strengthened glasshaving a high surface strength can be obtained. It is presumed thatsince the glass surface has some degree of surface roughness, stressconcentration is suppressed to enhance surface strength.

The surface roughness can be measured, for example, through a surfaceobservation with an AFM on a measuring area of 1 μm×1 μm.

Incidentally, conventional, chemically strengthened, unpolished glasssheets have a surface roughness of less than 0.20 nm.

[Measurement Conditions of AFM]

Apparatus: Nanoscope V+MultiMode 8 or Dimension ICON, manufactured byBruker GmbH

Mode: ScanAsyst mode

Probe: RTESPA (spring constant: 40 N/m)

Samples/line: 256

Lines: 256

Scan rate: 1 Hz

View to be measured: 1×1 μm² (targeting uncontaminated area)

<Method for Producing Chemically Strengthened Glass>

One embodiment of the method for producing a chemically strengthenedglass of the present invention is described below, to which, however,the present invention is not limited.

(Glass Composition)

Glass for use in the present invention may be any one containing sodium,and so far as it has a composition capable of being shaped andstrengthened through chemical strengthening treatment, various types ofcompositions can be used. Specifically, for example, there are mentionedan aluminosilicate glass, a soda-lime glass, a boron silicate glass, alead glass, an alkali barium glass, an aluminoborosilicate glass, etc.

The production method for a glass is not specifically limited. Desiredglass raw materials are put into a continuous melting furnace, and theglass raw materials are melted under heat at preferably from 1,500 to1,600° C., then refined and fed into a shaping device to shape themolten glass into a plate-like shape and gradually cooled to produce aglass.

Various methods may be employed for shaping a glass. For example,various shaping processes such as a down-draw process (for example, anoverflow down-draw process, a slot-down process, a redraw process,etc.), a float process, a roll-out process, and a pressing process maybe employed.

The thickness of a glass is not specifically limited, but foreffectively conducting chemical strengthening treatment, in general, thethickness thereof is preferably 5 mm or less, more preferably 3 mm orless.

The shape of a glass for use in the present invention is notspecifically limited. For example, various shapes of a glass such as aplate-like shape having a uniform thickness, a curved shape in which atleast one of the front surface or the back surface is curved, and athree-dimensional shape having a bend portion are employable.

Although the composition of the chemically strengthened glass of thepresent invention is not specifically limited, for example, thefollowing glass compositions may be mentioned.

(i) Glass having a composition including, in terms of mol %, from 50 to80% of SiO₂, from 2 to 25% of Al₂O₃, from 0 to 10% of Li₂O, from 0 to18% of Na₂O, from 0 to 10% of K₂O, from 0 to 15% of MgO, from 0 to 5% ofCaO and from 0 to 5% of ZrO₂.

(ii) Glass having a composition including, in terms of mol %, from 50 to74% of SiO₂, from 1 to 10% of Al₂O₃, from 6 to 14% of Na₂O, from 3 to11% of K₂O, from 2 to 15% of MgO, from 0 to 6% of CaO and from 0 to 5%of ZrO₂, in which the total content of SiO₂ and Al₂O₃ is 75% or less,the total content of Na₂O and K₂O is from 12 to 25%, and the totalcontent of MgO and CaO is from 7 to 15%.

(iii) Glass having a composition including, in terms of mol %, from 68to 80% of SiO₂, from 4 to 10% of Al₂O₃, from 5 to 15% of Na₂O, from 0 to1% of K₂O, from 4 to 15% of MgO and from 0 to 1% of ZrO₂.

(iv) Glass having a composition including, in terms of mol %, from 67 to75% of SiO₂, from 0 to 4% of Al₂O₃, from 7 to 15% of Na₂O, from 1 to 9%of K₂O, from 6 to 14% of MgO and from 0 to 1.5% of ZrO₂, in which thetotal content of SiO₂ and Al₂O₃ is from 71 to 75%, the total content ofNa₂O and K₂O is from 12 to 20%, and the content of CaO, if any, is lessthan 1%.

The chemically strengthened glass of the present invention has anion-exchanged compressive stress layer in the surface thereof. In theion exchange method, the surface of a glass is ion-exchanged to form asurface layer in which compressive stress remains. Specifically, thealkali metal ion (typically Li ion, Na ion) having a small ionic radiusin the surface of a glass sheet is substituted with an alkali ion havinga larger ionic radius (typically Na ion or K ion for Li ion, and K ionfor Na ion) through ion exchange at a temperature not higher than theglass transition point. Accordingly, compressive stress remains in thesurface of the glass, and the surface strength of the glass is therebyincreased.

In the production method of the present invention, chemicalstrengthening is conducted by bringing a glass into contact with aninorganic salt containing potassium nitrate (KNO₃). Accordingly, the Naion in the glass surface is ion-exchanged with the K ion in theinorganic salt to form a high-density compressive stress layer. Themethod for bringing a glass into contact with an inorganic salt includesa method of applying a pasty inorganic salt to a glass, a method ofspraying a glass with an aqueous solution of an inorganic salt, and amethod of immersing a glass in a salt bath of a molten salt heated at atemperature not lower than the melting point thereof, and of these, amethod of immersing in a molten salt is desirable.

The inorganic salt is preferably one having a melting point not higherthan the strain point of the glass to be strengthened (generally 500 to600° C.), and in the present invention, a salt containing potassiumnitrate (melting point: 330° C.) is preferred. Containing potassiumnitrate, the salt is preferred as capable of being in a molten state ata temperature not higher than the strain point of the glass and, inaddition, capable of being easily handled in the operating temperaturerange. The content of the potassium nitrate in the inorganic salt ispreferably 50% by mass or more.

Additionally, the inorganic salt preferably contains at least one saltselected from the group consisting of K₂CO₃, Na₂CO₃, KHCO₃, NaHCO₃,K₃PO₄, Na₃PO₄, K₂SO₄, Na₂SO₄, KOH and NaOH, and above all, morepreferably contains at least one salt selected from the group consistingof K₂CO₃, Na₂CO₃, KHCO₃ and NaHCO₃.

The above-mentioned salt (hereinafter this may be referred to as “fusingagent”) has a property of cutting the network of a glass typified by anSi—O—Si bond. Since the temperature at which chemical strengtheningtreatment is conducted is high such as a few hundred degrees C., thecovalent bond between Si—O in a glass is suitably cut at thattemperature and therefore the density-reducing treatment to be mentionedbelow for the glass can be easy to promote.

The degree of cutting the covalent bond may vary depending on the glasscomposition, the type of the salt (fusing agent) to be used, and thechemical strengthening treatment conditions such as the temperature andthe time, but is considered to be preferably selected from theconditions under which from 1 to 2 bonds of the four covalent bondsextending from Si can be cut.

For example, in a case where K₂CO₃ is used as a fusing agent, thecontent of the fusing agent in the inorganic salt is 0.1 mol % or moreand the chemical strengthening treatment temperature is from 350 to 500°C., the chemical strengthening treatment time is preferably from 1minute to 10 hours, more preferably from 5 minutes to 8 hours, even morepreferably from 10 minutes to 4 hours.

The amount of the fusing agent to be added is, from the viewpoint ofsurface hydrogen concentration control, preferably 0.1 mol % or more,more preferably 1 mol % or more, and particularly preferably 2 mol % ormore. From the viewpoint of productivity, the amount thereof ispreferably not larger than the saturation solubility of each salt. Whenthe fusing agent is excessively added, there is a concern of causingglass corrosion. For example, in a case where K₂CO₃ is used as thefusing agent, the amount thereof is preferably 24 mol % or less, morepreferably 12 mol % or less, particularly preferably 8 mol % or less.

The inorganic salt may contain any other chemical species within a rangenot impairing the advantageous effects of the present invention, inaddition to potassium nitrate and the fusing agent. For example, thereare mentioned alkali chloride salts or alkali borate salts such assodium chloride, potassium chloride, sodium borate, and potassiumborate. One or more of these may be added either singly or as combined.

The production method of the present invention is described below withreference to examples of an embodiment in which chemical strengtheningis performed according to a method of immersing a glass in a moltensalt.

(Production of Molten Salt 1)

A molten salt may be produced according to steps mentioned below.

Step 1a: Preparation of potassium nitrate molten salt

Step 2a: Addition of fusing agent to the potassium nitrate molten salt

(Step 1a—Preparation of Potassium Nitrate Molten Salt—)

In the step 1a, potassium nitrate is put into a container, and melted byheating at a temperature not lower than the melting point thereof toprepare a molten salt. The melting is conducted at a temperature fallingwithin a range of from the melting point (330° C.) of potassium nitrateto the boiling point (500° C.) thereof. In particular, it is morepreferable that the melting temperature is from 350 to 470° C. from theviewpoint of the balance between the surface compressive stress (CS) tobe given to a glass and the depth of the compressive stress layer (DOL)and of the strengthening time.

Regarding the container for melting potassium nitrate, metals, quartz,ceramics and the like can be used. Above all, from the viewpoint ofdurability, metal materials are desirable, and from the viewpoint ofcorrosion resistance, stainless steel (SUS) materials are preferred.

(Step 2a—Addition of Fusing Agent to the Potassium Nitrate Molten Salt—)

In the step 2a, the above-mentioned fusing agent is added to thepotassium nitrate molten salt prepared in the step 1a, and, while keptat a temperature falling within a certain definite range, mixed with animpeller or the like so that the whole becomes uniform. In a case whereplural fusing agents are used, the order of adding them is notspecifically limited, and these may be added at a time.

The temperature is preferably not lower than the melting point ofpotassium nitrate, that is, preferably 330° C. or higher, morepreferably from 350 to 500° C. The stirring time is preferably from 1minute to 10 hours, more preferably from 10 minutes to 2 hours.

(Production of Molten Salt 2)

In the above-mentioned production of molten salt 1, a method of adding afusing agent after preparation of a molten salt of potassium nitrate isexemplified, but apart from the method, the molten salt may also beproduced according to the following steps.

Step 1b: mixing of potassium nitrate and fusing agent

Step 2b: melting of mixed salt of potassium nitrate and fusing agent

(Step 1b—Mixing of Potassium Nitrate and Fusing Agent—)

In the step 1b, potassium nitrate and a fusing agent are put into acontainer and mixed with an impeller or the like. In a case where pluralfusing agents are used, the order of adding them is not specificallylimited, and these may be added at a time. The container to be used maybe the same one as that to be used in the above-mentioned step 1a.

(Step 2b—Melting of Mixed Salt of Potassium Nitrate and Fusing Agent—)

In the step 2b, the mixed salt obtained in the step 1b is melted byheating. The melting is conducted at a temperature falling within arange of from the melting point (330° C.) of potassium nitrate to theboiling point (500° C.) thereof. In particular, it is more preferablethat the melting temperature is from 350 to 470° C. from the viewpointof the balance between the surface compressive stress (CS) to be givento a glass and the depth of the compressive stress layer (DOL) and ofthe strengthening time. The stirring time is preferably from 1 minute to10 hours, more preferably from 10 minutes to 2 hours.

In a case where sediments form in the molten salt obtained through theabove-mentioned step 1a and the step 2a, or through the step 1b and thestep 2b, by adding a fusing agent thereto, the molten salt is keptstatically until the sediments precipitate in the bottom of thecontainer, before the chemical strengthening treatment for a glass. Thesediments contain the fusing agent exceeding the saturation solubilitythereof, and salts formed through exchange of cation in the fusing agentin the molten salt.

The molten salt for use in the production method of the presentinvention has an Na concentration of preferably 500 ppm by weight ormore, more preferably 1,000 ppm by weight or more. The Na concentrationof 500 ppm by weight or more in the molten salt is preferred since thelow-density layer can easily deepen in the acid treatment step to bementioned hereinunder. The upper limit of the Na concentration is notspecifically defined, and is acceptable to a level at which a desiredsurface compressive stress (CS) can be obtained.

The molten salt used for chemical strengthening treatment once or morecontains sodium released from a glass. Therefore, when the Naconcentration is already within the above-mentioned range, glass-derivedsodium may be used as such for the Na source, or when the Naconcentration is insufficient or when a fresh molten salt that has notbeen used for chemical strengthening treatment is used, the Naconcentration may be controlled by adding an inorganic sodium salt suchas sodium nitrate.

As described above, a molten salt can be prepared according to theabove-mentioned step 1a and the step 2a, or the step 1b and the step 2b.

(Chemical Strengthening)

Next, using the prepared molten salt, chemical strengthening treatmentis performed. In the chemical strengthening treatment, a glass isimmersed in a molten salt and the metal ion (Na ion) in the glass issubstituted with a metal ion (K ion) having a larger ionic radius in themolten salt. Through the ion exchange, the composition of the glasssurface is changed, and the glass surface is densified to form acompressive stress layer 20 [(a) to (b) in FIG. 2]. The densification ofthe glass surface generates compressive stress to strengthen the glass.

In fact, the density of chemically strengthened glass graduallyincreases from the outer edge of the interlayer 30 (bulk) existing inthe center of the glass toward the surface of the compressive stresslayer, and therefore between the interlayer and the compressive stresslayer, there exists no definite boundary at which the density suddenlychanges. Here, the interlayer means a layer existing in the center partof the glass and surrounded by the compressive stress layer. Theinterlayer is a layer not undergone ion exchange, differing from thecompressive stress layer.

Specifically, the chemical strengthening treatment in the presentinvention is performed by the following step 3.

Step 3: Chemical Strengthening Treatment for Glass

(Step 3—Chemical Strengthening Treatment for Glass—)

In the step 3, a glass is preheated, and the temperature of the moltensalt prepared in the above-mentioned step 1a and the step 2a or the step1b and the step 2b is adjusted to a temperature for chemicalstrengthening. Next, the preheated glass is immersed in the molten saltfor a predetermined period of time, then the glass is drawn up from themolten salt and left cooled. Preferably, prior to the chemicalstrengthening treatment, the glass is processed for shaping inaccordance with the use thereof, for example, through mechanicalprocessing such as cutting, end surface machining, drilling, etc.

The glass preheating temperature depends on the temperature at which theglass is immersed in a molten salt, but, in general, preferably 100° C.or higher.

The chemical strengthening temperature is preferably not higher than thestrain point of the glass to be strengthened (generally 500 to 600° C.),but for obtaining a greater compressive stress layer depth, particularlypreferably 350° C. or higher.

The immersion time for the glass in a molten salt is preferably from 1minute to 10 hours, more preferably from 5 minutes to 8 hours, even morepreferably from 10 minutes to 4 hours. Falling within the range, it ispossible to obtain a chemically strengthened glass excellent in thebalance between the surface strength and the depth of the compressivestress layer.

In the production method of the present invention, the following stepsare performed after the chemical strengthening treatment.

Step 4: Washing of the Glass

Step 5: Acid Treatment of the Glass after Step 4

At the time after the above-mentioned step 5, the glass surface furtherhas a low-density layer 10 in which the surface layer of the compressivestress layer 20 has been denatured, specifically, the density thereofhas been reduced [(b) to (c) in FIG. 2]. The low-density layer is formedthrough leaching of Na and K from the outermost surface of thecompressive stress layer, and in place of these, H has penetrated(substituted) therein.

The step 4 and the step 5 are described in detail hereinunder.

(Step 4—Washing of Glass—)

In the step 4, the glass is washed with industrial water, ion-exchangedwater or the like. Above all, ion-exchanged water is preferred. Thewashing condition may vary depending on the washing liquid to be used,but in a case where ion-exchanged water is used, it is preferable thatthe glass is washed at 0 to 100° C. from the viewpoint of completelyremoving the adhered salts.

(Step 5—Acid Treatment—)

In the step 5, the glass washed in the step 4 is further subjected to anacid treatment.

In the acid treatment for a glass, a chemically strengthened glass isimmersed in an acidic solution, whereby Na and/or K in the surface ofthe chemically strengthened glass can be substituted with H.

The solution is not specifically limited so far as it is acidic and hasa pH of less than 7, in which the acid to be used may be a weak acid ora strong acid. Specifically, the acid is preferably hydrochloric acid,sulfuric acid, phosphoric acid, acetic acid, oxalic acid, carbonic acid,citric acid, etc. These acids may be used either singly or as combined.

The temperature for performing the acid treatment varies depending onthe type and the concentration of the acid to be used and the treatingtime, but is preferably 100° C. or lower.

The time for performing the acid treatment also varies depending on thetype, the concentration and the temperature of the acid to be used, butis preferably from 10 seconds to 5 hours from the viewpoint ofproductivity, more preferably from 1 minute to 2 hours.

The concentration of the solution for performing the acid treatmentvaries depending on the type and the temperature of the acid to be usedand the treating time, but is preferably a concentration in which riskof container corrosion is less, and specifically, the concentrationthereof is preferably from 0.1 wt % to 20 wt %.

The low-density layer is removed in alkali treatment to be mentionedbelow, and a thicker low-density layer is preferred as the glass surfaceis easy to remove. Accordingly, the thickness of the low-density layeris preferably 5 nm or more from the viewpoint of the amount of glasssurface removal, more preferably 20 nm or more. The thickness of thelow-density layer may be controlled by controlling the fusing agentconcentration, the sodium concentration, the temperature, the time andthe like in the chemical strengthening step.

The density of the low-density layer is preferably lower than thedensity in the region (bulk) deeper than the ion-exchanged compressivestress layer, from the viewpoint the glass surface removability.

The thickness of the low-density layer may be determined from the period(AO) measured in X-ray reflectometry (XRR).

The density of the low-density layer may be determined from the criticalangle (θc) measured in XRR.

In a simplified manner, formation of a low-density layer and thethickness of the layer may be confirmed through observation of the crosssection of a glass with a scanning electronic microscope (SEM).

In the production method of the present invention, the following step isperformed after the acid treatment.

Step 6: Alkali Treatment

In the step 6, a part or all of the low-density layer 10 formed up tothe step 5 may be removed [(c) to (d) in FIG. 2].

The step 6 is described in detail hereinunder.

(Step 6—Alkali Treatment—)

In the step 6, the glass having been subjected to the acid treatment inthe step 5 is further subjected to an alkali treatment.

In the alkali treatment, the chemically strengthened glass is immersedin a basic solution, whereby a part or all of the low-density layer isremoved.

The solution is not specifically limited so far as it is basic and has apH of more than 7, in which any of a weak base or a strong base isusable. Specifically, a base such as sodium hydroxide, potassiumhydroxide, potassium carbonate, sodium carbonate or the like ispreferred. These bases may be used either singly or as combined.

The temperature for performing the alkali treatment varies depending onthe type and the concentration of the base to be used and the treatingtime, but is preferably from 0 to 100° C., more preferably from 10 to80° C., even more preferably from 20 to 60° C. The temperature range ispreferred as causing no risk of glass corrosion.

The time for performing the alkali treatment also varies depending onthe type, the concentration and the temperature of the base to be used,but is preferably from 10 seconds to 5 hours from the viewpoint ofproductivity, more preferably from 1 minute to 2 hours.

The concentration of the solution for performing the alkali treatmentvaries depending on the type and the temperature of the base to be usedand the treating time, but is preferably from 0.1 wt % to 20 wt % fromthe viewpoint of glass surface removability.

Through the above-mentioned alkali treatment, a part or all of thelow-density layer with H having penetrated thereinto is removed and thesurface layer in which the hydrogen concentration profile satisfies thespecific relational equation (I) described above is exposed out.Accordingly, a chemically strengthened glass having an improved surfacestrength can be obtained. Further, since the low-density layer isremoved, the flaws existing in the glass surface are also removed at thesame time. Therefore, it is considered that this point also contributesto the surface strength improvement.

Between the above-mentioned acid treatment step 5 and the alkalitreatment step 6, or after the alkali treatment step 6, it is preferableto perform a washing step like the step 4.

In the production method of the present invention, the chemical liquidsto be handled are highly safe and therefore the method does not requireany special equipment. Accordingly, a chemically strengthened glasswhose surface strength has dramatically increased can be obtained safelyand efficiently.

The amount of the low-density layer to be removed depends on the alkalitreatment condition. An embodiment in which the low-density layer 10 hasbeen completely removed is shown in (d) of FIG. 2, however, a part ofthe low-density layer 10 may be removed while a part thereof hasremained. From the viewpoint of surface strength improvement, the effectcan be obtained even when not all the low-density layer is removed, butfrom the viewpoint of stably securing the transmittance of glass, it ispreferable that all the low-density layer is removed.

EXAMPLES

The present invention is described specifically with reference toExamples given below, but the present invention is not limited thereto.

<Evaluation Method>

Various evaluations in present Examples were performed according to theanalysis methods mentioned below.

(Evaluation of Glass: Surface Stress)

The compressive stress value of the compressive stress layer and thedepth of the compressive stress layer in the chemically strengthenedglass of the present invention can be measured using EPMA (electronprobe microanalyzer) or a surface stress meter (for example, FSM-6000manufactured by Orihara Manufacturing Co., Ltd.), etc. In Examples, thesurface compressive stress value (CS, unit: MPa) and the depth of thecompressive stress layer (DOL, unit: μm) were measured using a surfacestress meter (FSM-6000) manufactured by Orihara Manufacturing Co., Ltd.

(Evaluation of Glass: Removal Amount)

The removal amount thickness of a glass was determined by measuring theweight thereof before and after chemical liquid treatment, using ananalytical electronic balance (HR-202i, manufactured by A&D Company,Limited), and converting the found value into a thickness according tothe following equation.(Removal amount thickness per one surface)=((weight beforetreatment)−(weight after treatment))/(glass specific gravity)/treatedarea/2At this time, the calculation was made while defining the glass specificgravity as 2.48 (g/cm³).(Evaluation of Glass: Surface Strength)

The glass surface strength was measured according to the ball-on-ring(BOR) test. FIG. 1 shows a schematic view for explaining theball-on-ring test employed in the present invention. A glass sheet 1was, while kept set horizontally, pressurized by a pressurizing jig 2made of SUS304 (hardened steel, diameter 10 mm, mirror-finished) tomeasure the surface strength of the glass sheet 1.

In FIG. 1, the glass sheet 1 to be a sample is horizontally disposed ona receiving jig 3 made of SUS304 (diameter: 30 mm, curvature radius ofthe contact part R: 2.5 mm, the contact part is hardened steel,mirror-finished). Above the glass sheet 1, a pressurizing jig 2 forpressurizing the glass sheet 1 is arranged.

In this embodiment, the center region of the glass sheet 1 obtained inExamples and Comparative Examples was pressurized from the above of theglass sheet 1. The test condition is as mentioned below.

Descending rate of pressurizing jig 2: 1.0 (mm/min)

In this test, the fracture load (unit: N) at which the glass wasfractured was taken as a BOR surface strength. The average value oftwenty measured values thereof was taken as a surface strength F.However, in a case where the fracture origin of the glass sheet wasseparated from a loading point of the ball by 2 mm or more, the obtainedvalue was excluded from the data for calculating the average value.

(Evaluation of Glass: Surface Roughness)

The surface roughness of a glass was measured using an AFM under thefollowing conditions.

[Measurement Conditions of AFM]

Apparatus: Nanoscope V+MultiMode 8 or Dimension ICON, manufactured byBruker GmbH

Mode: ScanAsyst mode

Probe: RTESPA (spring constant: 40 N/m)

Samples/line: 256

Lines: 256

Scan rate: 1 Hz

View to be measured: 1×1 μm² (targeting uncontaminated area)

(Evaluation of Glass: Hydrogen Concentration)

According to the method described in the section of [Method forMeasuring Hydrogen Concentration Profile] given hereinabove, thehydrogen concentration profile was determined and the relationalequation (I) and an average hydrogen concentration (c value) werederived therefrom.

Example 1

(Chemically Strengthening Step)

In a SUS-made cup, 9,700 g of potassium nitrate, 890 g of potassiumcarbonate and 400 g of sodium nitrate were introduced, and the contentswere heated to 450° C. with a mantle heater to prepare a molten saltcontaining 6 mol % of sodium carbonate and 10,000 ppm by weight ofsodium. Aluminosilicate glass A (specific gravity: 2.48) havingdimensions of 50 mm×50 mm×0.56 mm was prepared, preheated at 200 to 400°C., and subjected to a chemical strengthening treatment by immersing inthe molten salt at 450° C. for 2 hours for ion exchange and cooling toaround room temperature. The resultant chemically strengthened glass waswashed with water and subjected to the next step.

Composition of the aluminosilicate glass A (in terms of mol %): SiO₂64.4%, Al₂O₃ 8.0%, Na₂O 12.5%, K₂O 4.0%, MgO 10.5%, CaO 0.1%, SrO 0.1%,BaO 0.1%, ZrO₂ 0.5%

(Acid Treatment Step)

In a beaker, 13.4 wt % hydrochloric acid (HCl, manufactured by KantoChemical Co., Inc.) was prepared, and the temperature thereof wasadjusted to 41° C. using a water bath. The glass obtained in thechemical strengthening step was immersed in the hydrochloric acid whosetemperature was adjusted, for 180 seconds to perform acid treatment, andthen washed with pure water several times, and thereafter dried with airblowing. The resultant glass was subjected to the next step.

(Alkali Treatment Step)

An aqueous solution of 4.0 wt % sodium hydroxide was prepared in abeaker, and the temperature thereof was adjusted to 40° C. using a waterbath. The glass obtained in the acid treatment step was immersed in thepotassium hydroxide aqueous solution whose temperature was adjusted, for120 seconds to perform alkali treatment, and then washed with pure waterseveral times, and thereafter dried with air blowing.

Thus, a chemically strengthened glass of Example 1 was obtained.

Example 2

A chemically strengthened glass was produced in the same manner as inExample 1, except that an aluminosilicate glass A having the sheetthickness shown in Table 1 was used, that the temperature of the moltensalt and the ion exchange treatment time were changed to 430° C. and 40minutes, respectively, and that an acid treatment and an alkalitreatment were performed by showering the prepared aqueous hydrochloricacid solution and aqueous sodium hydroxide solution on the glass for 277seconds each using a glass washer of the type in which glass sheets werehorizontally held and conveyed.

Example 3

A chemically strengthened glass was produced in the same manner as inExample 1, except that an aluminosilicate glass B (specific gravity:2.41) having dimensions of 50 mm×50 mm×0.72 mm and having the followingcomposition was used in place of the aluminosilicate glass A, that inthe acid treatment step, 6.0 wt % nitric acid (HNO₃; manufactured byKanto Chemical Co., Inc.) was prepared in a resinous tank and thetemperature thereof was adjusted to 41° C. using a fluororesin-coatedheater (KKS14A, manufactured by Hakko Electric Co. Ltd.), and that theglass was immersed for 120 seconds in the nitric acid whose temperaturewas adjusted, thereby performing an acid treatment.

Composition of aluminosilicate glass B (in terms of mol %): SiO₂ 68%,Al₂O₃ 10%, Na₂O 14%, MgO 8%

Example 4

A chemically strengthened glass was produced in the same manner as inExample 3, except that an aluminoborosilicate glass (specific gravity:2.38) having dimensions of 50 mm×50 mm×0.70 mm and having the followingcomposition was used in place of the aluminosilicate glass A.

Composition of the aluminoboro silicate glass (in terms of mol %): SiO₂67%, B₂O₃ 4%, Al₂O₃ 13%, Na₂O 14%, K₂O<1%, MgO 2%, CaO<1%

Comparative Example 1

A chemically strengthened glass was produced in the same manner as inExample 1, except that the sodium amount in the molten salt in thechemical strengthening step was changed to a value shown in Table 1,that the addition amount of potassium carbonate was changed to 0 g, andthat the acid treatment step and the alkali treatment step were omitted.

Comparative Example 2

A chemically strengthened glass was produced in the same manner as inExample 2, except that the sodium amount in the molten salt in thechemical strengthening step was changed to a value shown in Table 1,that the addition amount of potassium carbonate was changed to 0 g, andthat the acid treatment step and the alkali treatment step were omitted.

Comparative Example 3

A chemically strengthened glass was produced in the same manner as inExample 3, except that the sodium amount in the molten salt in thechemical strengthening step was changed to a value shown in Table 1,that the addition amount of potassium carbonate was changed to 0 g, andthat the acid treatment step and the alkali treatment step were omitted.

The thus-obtained chemically strengthened glass was evaluated forvarious properties. The results are shown in Table 1.

FIG. 3 to FIG. 5 show graphs in which the hydrogen concentration profilein the surface layer of each chemically strengthened glass obtained inExamples 1 to 4 and Comparative Examples 1 to 3 was plotted.

Further, FIG. 8 shows a Weibull plot of BOR surface strength evaluationof each of chemically strengthened glasses obtained in Example 1 andComparative Example 1. FIG. 8 shows a Weibull plot of BOR surfacestrength evaluation of an aluminosilicate glass sheet sample having athickness of 0.56 mm. The horizontal axis of the graph indicates alogarithm ln (σ) of the fracture load σ (N), and the vertical axisthereof indicates a cumulative fracture probability percentage P (%)relative to the sample in each of the two groups.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Type of Glassaluminosilicate aluminosilicate aluminosilicate Aluminoboro- glass Aglass A glass B silicate glass Chemical K₂CO₃ mol % 6 6 6 6strengthening Na content wt ppm 10,000 4,000 10,000 10,000 Temperature °C. 450 430 450 450 Time min 120 40 120 120 Acid Chemical liquid type HClHCl HNO₃ HNO₃ treatment Concentration wt % 13.4 13.4 6 6 Temperature °C. 41 41 41 41 Time sec 180 277 120 120 Alkali Chemical liquid type NaOHNaOH NaOH NaOH treatment Concentration wt % 4 4 4 4 Temperature ° C. 4040 40 40 Time sec 120 277 120 120 Sheet thickness mm 0.56 0.71 0.72 0.70Sheet strength N 871 1101 1362 1053 X: F > Xt² 2777 2165 2650 2179Surface removal amount nm 154 50 159 186 Surface n = 1 nm 0.33 0.33 0.240.46 roughness n = 2 nm 0.39 0.31 0.24 0.46 Expression a −0.159 −0.268−0.099 −0.198 (I) b 0.138 0.202 0.133 0.153 Average c mol/L 0.099 0.1350.109 0.103 hydrogen (as H₂O) concentration CS MPa 666 759 881 756 DOLμm 39 22 28 35 Polishing flaws Absent Absent Absent Absent ComparativeComparative Comparative Reference Example 1 Example 2 Example 3 Example1 Type of Glass aluminosilicate aluminosilicate Aluminoboro-aluminosilicate glass A glass A silicate glass glass B Chemical K₂CO₃mol % 0 0 0 0 strengthening Na content wt ppm 4,000 4,000 2,000 2,000Temperature ° C. 450 430 450 450 Time min 120 40 120 120 Acid Chemicalliquid type — — — — treatment Concentration wt % — — — — Temperature °C. — — — — Time sec — — — — Alkali Chemical liquid type — — — —treatment Concentration wt % — — — — Temperature ° C. — — — — Time sec —— — — Sheet thickness mm 0.56 0.72 0.68 0.70 Sheet strength N 455 687413 564 X: F > Xt² 1432 1333 858 1151 Surface removal amount nm — — — —Surface n = 1 nm 0.19 0.19 0.19 0.44 roughness n = 2 nm 0.17 0.19 0.170.49 Expression a −0.295 −0.219 −0.274 −0.305 (I) b 0.311 0.181 0.2290.148 Average c mol/L 0.238 0.128 0.161 0.073 hydrogen (as H₂O)concentration CS MPa 707 679 779 949 DOL μm 45 20 37 27 Polishing flawsAbsent Absent Absent Present

The results given in Table 1 show that Examples 1 to 4, which had asurface roughness (Ra) of 0.20 nm or higher and satisfied relationalexpression (I), were greatly improved in the surface strength ascompared to Comparative Examples 1 to 3.

The results given in FIG. 8 show that the average fracture load ofExample 1 was 827 N, whereas that of Comparative Example 1 was 455 N.The 10% fracture load (B10) of Example 1 was 793 N, whereas that ofComparative Example 1 was 318 N. The 1% fracture load (B1) of Example 1was 750 N, whereas that of Comparative Example 1 was 200 N. It can beseen from these results that Example 1 does not produce low-strengthproducts, and the products obtained have greatly improved reliabilityfor the surface strength.

Reference Example 1

Cerium oxide having an average particle diameter of 1.2 μm was dispersedin water to produce a slurry having a specific gravity of 0.9. The samealuminosilicate glass B as in Example 3 was polished therewith in anamount of 0.6 μm per surface using a polishing pad (nonwoven fabrictype) under a polishing pressure of 10 kPa. The glass obtained throughthe polishing step was chemically strengthened using the same moltensalt as in Comparative Example 3 at a chemical strengthening temperatureof 450° C. for a chemical strengthening treatment time of 2 hours. Thischemically strengthened glass was evaluated for various properties, andthe results thereof are shown in Table 1. An image obtained by observinga surface of this glass with an AFM is shown in FIG. 9. The surfaceroughness (Ra) thereof measured by the AFM measurement was 0.40 nm.

Meanwhile, a surface observation image of the chemically strengthenedglass produced in Example 1 is shown in FIG. 10. The surface roughness(Ra) thereof measured by the AFM measurement was 0.33 nm.

Reference Example 2

The same aluminosilicate glass B as in Example 3 was chemicallystrengthened using the same molten salt as in Comparative Example 3 at achemical strengthening treatment temperature of 450° C. for a chemicalstrengthening treatment time of 2 hours. The glass which had beenchemically strengthened was etched in an amount of 1.06 μm per surfaceby immersing the glass in a 25° C. solution composed of hydrofluoricacid having a concentration of 1.0 wt % and hydrochloric acid having aconcentration of 18.5 wt % for 60 seconds.

FIG. 11 shows Weibull plots of the BOR surface strength evaluation ofthe chemically strengthened glasses obtained in Reference Example 2 andExample 3. The average fracture load of Example 3 was 1,362 N, whereasthat of Reference Example 2 was 1,266 N. The 10% fracture load (B10) ofExample 3 was 1,339 N, whereas that of Reference Example 2 was 1,098 N.The 1% fracture load (B1) of Example 3 was 1,307 N, whereas that ofReference Example 2 was 904 N. It can be seen from these results thatExample 3 does not produce low-strength products, and the productsobtained have greatly improved reliability for the surface strength.

Furthermore, FIG. 12 shows the hydrogen concentration profiles ofsurface layers of the chemically strengthened glasses obtained inReference Example 2 and Example 3. Example 3 was higher in surface-layerhydrogen concentration than Reference Example 2. The average hydrogenconcentration c of the near-surface region (depth X from the outermostsurface was 0.1 to 0.4 μm) in Reference Example 2 was lower than theaverage hydrogen concentrations c in Examples 1 to 4. It is hencepresumed that the improvement in reliability for surface strength is dueto the slightly high hydrogen (moisture) concentration in the vicinityof the glass surface layer, although the reason therefor in uncertain.

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof. The presentapplication is based on a Japanese patent application filed on Jul. 19,2013 (Japanese Patent Application No. 2013-151116), and the contentthereof is herein incorporated by reference.

INDUSTRIAL APPLICABILITY

According to the present invention, a chemically strengthened glasswhose surface strength has been dramatically improved can be obtainedsafely and inexpensively. The chemically strengthened glass of thepresent invention is usable as a cover glass for displays such as mobilephones, digital cameras, and touch panel displays.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

-   -   10: Low-density layer    -   20: Compressive stress layer    -   30: Interlayer

The invention claimed is:
 1. A glass sheet, having a thickness of 5 mmor less and having a compressive stress layer formed in a surface layerthereof according to an ion exchange method, wherein the glass sheet hasa surface roughness (Ra) of 0.20 nm or higher, a hydrogen concentrationY in a region to a depth X from a surface of the glass sheet satisfiesrelational equation (I) at X=from 0.1 to 0.4 (μm), a surface strength F(N) measured by a ball-on-ring test is at least a value of (1500×t²),wherein t (mm) is thickness of the glass sheet, and two or morescratches each having a length of 5 μm or more and a width of 0.1 μm ormore are not present in a region of 10 μm×5 μm of the surface of theglass sheet:Y=aX+b  (I) wherein: Y is the hydrogen concentration measured as H₂O,(mol/L); X is the depth from the surface of the glass sheet (m); a is ina range from −0.270 to −0.005; and b is in a range from 0.020 to 0.220,wherein the ball-on-ring test is conducted by: placing the glass sheeton a stainless ring whose diameter is 30 mm and whose contact part has aroundness with a curvature radius of 2.5 mm; applying a load on a centerof the ring by the ball under a static loading condition, while a steelball having a diameter of 10 mm is kept in contact with the glass sheet;and measuring a BOR surface strength which is a fracture load (unit: N)at which the glass sheet is fractured and determining an average valueof twenty measured values of the BOR surface strength as the surfacestrength F, provided that in a case where a fracture origin of the glasssheet is apart from a loading point of the ball by 2 mm or more, theobtained value is excluded from data for calculating the average value.2. The glass sheet according to claim 1, wherein the glass sheet is madeof an aluminosilicate glass, an aluminoborosilicate glass or a soda-limeglass.
 3. The glass sheet according to claim 1, wherein the thickness ofthe glass sheet is 3 mm or less.
 4. The glass sheet according to claim1, wherein in the relational equation (I), a is in a range from −0.240to −0.030.
 5. The glass sheet according to claim 1, wherein in therelational equation (I), a is in a range from −0.210 to −0.050.
 6. Theglass sheet according to claim 1, wherein in the relational equation(I), b is in a range from 0.020 to 0.215.
 7. The glass sheet accordingto claim 1, wherein in the relational equation (I), b is in a range from0.030 to 0.210.
 8. The glass sheet according to claim 1, wherein in therelational equation (I), b is in a range from 0.040 to 0.200.
 9. Theglass sheet according to claim 1, wherein in the relational equation(I), a is in a range from −0.210 to −0.050 and b is in a range from0.040 to 0.200.
 10. The glass sheet according to claim 1, wherein anaverage hydrogen concentration, at a depth from the surface of the glasssheet of from 0.1 μm to 0.4 μm, is in a range of from 0.070 to 0.150mol/L.
 11. The glass sheet according to claim 1, wherein the surfacestrength F (N) is at least a value of (1800×t²).
 12. The glass sheetaccording to claim 1, wherein the glass sheet is made of analuminoborosilicate glass.
 13. The glass sheet according to claim 1,wherein the glass sheet is not etched by hydrofluoric acid.
 14. A coverglass of a display, comprising the glass sheet according to claim
 1. 15.The glass sheet according to claim 1, wherein the glass sheet isobtained by a process comprising: immersing a glass sheet in a moltensalt comprising potassium nitrate to ion-exchange the glass sheet;immersing the ion-exchanged glass sheet in an acid solution to subjectthe glass sheet to an acid treatment; and immersing the glass sheetwhich is subjected to the acid treatment in a basic solution.
 16. Theglass sheet according to claim 15, wherein the molten salt furthercomprises at least one fusing agent selected from the group consistingof K₂CO₃, Na₂CO₃, KHCO₃, NaHCO₃, K₃PO₄, Na₃PO₄, K₂SO₄, Na₂SO₄, KOH andNaOH.
 17. The glass sheet according to claim 15, wherein an Naconcentration in the molten salt is 500 ppm or more.
 18. The glass sheetaccording to claim 15, wherein an Na concentration in the molten salt is1,000 ppm or more.
 19. The glass sheet according to claim 18, whereinthe acid solution comprises at least one acid selected from the groupconsisting of hydrochloric acid, sulfuric acid, phosphoric acid, aceticacid, oxalic acid, carbonic acid, and citric acid.
 20. The glass sheetaccording to claim 19, wherein the basic solution comprises at least onebase selected from the group consisting of sodium hydroxide, potassiumhydroxide, potassium carbonate, and sodium carbonate.